nova: Release Train 20.6.0

Minor version bump is due to patch 62bf0ef129 that modifies the allowed
value of a configuration parameter.

$ git log --oneline --no-merges 20.5.0..HEAD
ff570d1b4e [stable-only] gate: Pin CEPH_RELEASE to nautilus in LM hook
3c8841595a tools: Allow check-cherry-picks.sh to be disabled by an env var
e9e0998978 Handle disabled CPU features to fix live migration failures
62bf0ef129 Disallow CONF.compute.max_disk_devices_to_attach = 0
4839d41fc0 Use cell targeted context to query BDMs for metadata
b6cc7e9aac Fix a hacking test
cd83da5501 libvirt: Only ask tpool.Proxy to autowrap vir* classes

Change-Id: I87bc36c4951ff85fdab0f02424b216feb43bb944

deliverables/train/nova.yaml

@@ -95,6 +95,10 @@ releases:
     projects:
       - repo: openstack/nova
         hash: 8378785f995dd4bec2a5a20f7bf5946b3075120d
+  - version: 20.6.0
+    projects:
+      - repo: openstack/nova
+        hash: ff570d1b4e9b9777405ae75cc09eae2ce255bf19
 branches:
   - name: stable/train
     location: 20.0.0.0rc1